In order to respond to the problems of the lack of available frequency bands, notably in the UHF band explained above, it appears interesting for broadband systems and narrow band systems to be housed within a same frequency band. Hence, by considering a broadband system with a 5 MHz channel width, whose operating would preferably be located in UHF band, it would seem interesting if, within said 5 MHz band, one or several narrow band communication systems were present. If, for example, the channel, or pipe, width of the narrow band communication systems is 12.5 kHz, this implies that 400 narrow band channels are likely to be present in a 5 MHz broadband channel, which makes the situation particularly complicated to manage. However, this situation, in reality, is much simpler, the number of narrow band channels actually used being less than the number of useable channels. Indeed, in a given place of the network coverage and at a given time, the full amount of the narrow band channels are not all used, or even, out of at least a certain number of these narrow band channels, the narrow band signal level is relatively low.
In practice, in order to respect the cellular organisation of most of the narrow band systems, a re-use factor is defined for reutilising the communication channels, which takes account of the fact that in two adjacent cells, notably, the same communication channel is not used for each of the considered cells in order to essentially reduce the risks of interference.
Hence, when considering a narrow band communication system implemented using a re-use factor of N, a number of narrow band channels used in each cell are obtained within a considered 5 MHz frequency band equal to: (5 MHz/N)/Bnarrow band, where Bnarrow band corresponds to the width of the narrow band channels considered.
For example, for a re-use factor of 20, and a narrow band channel width, Bnarrow band, of 12.5 kHz, each cell comprises (5 MHz/20)/12.5 kHz=20 narrow band channels used within the cell out of a total of 400 narrow band channels theoretically useable in the 5 MHz mode. FIG. 1 illustrates such a situation, with the narrow band channels 101 implemented within a 5 MHz frequency band 102 used by a broadband system. Even if all narrow band channels are not used, the coexistence within certain frequency ranges of broadband system signals and of narrow band system signals give rise to mutual interferences between these signals. The broadband system signals more or less interfere with each other, depending on the density of narrow band channels.
Hence, when considering the example in FIG. 1, in which 20 narrow band channels 101 are present within the 5 Mz band of the broadband channel 102, a receptor of the broadband system receives the broadband signal in addition to the twenty narrow band signals. Nevertheless, the power of these narrow band signals may be high, for example, the power of each one may be of the same type or similar to that of the broadband signal. The emission levels of mobile terminals are in fact limited, whatever the band width, notably for reasons of autonomy. In the example of FIG. 1, when considering the assumption according to which the power received for each of the narrow band channels is equal to, or near to, that received for a broadband signal, the signal ratio on interference for the broadband signal is −10*log 10 (20)=−13 dB. Such signal ratio on interference is very unfavourable and the broadband signal is most likely not to be able to be correctly processed under such conditions. These conditions, and even more difficult conditions, may be commonly encountered, for example, in the case of using, on the same location and thus on the same coverage, a broadband system and a narrow band system, or even a broadband system and several narrow band systems.